Hi!
I'd really recommend that you at least scan IFA11 and IFA14 from ETSI, those documents are the foundation for the NFVO models.
The NFVO packages also bundle some documentation and examples that shows how to use the NFVO.
Further than that, I have a couple of videos here that walks through:
VNFD: https://youtu.be/yc4EQvCii9k
NSD: NFVO-NSD - YouTube
Instantiation through NS info: NFVO-NS-info - YouTube